Amino acids as green neutralizing agent for acidic corrosion inhibitors

ABSTRACT

With the present invention, sustainable systems for corrosion, inhibition in the processing of metals (in particular iron, aluminum and magnesium) are made available. These systems being in full accordance with the principles of green chemistry comprise an amino acid as neutralizing component for an acidic corrosion inhibitor, whereby the amino acid is used in deprotonated form. The resulting metalworking fluids may be water- or oil-based or semi synthetic formulations. The neutralization of acidic corrosion inhibitors with the claimed system comprising an amino acid consistently achieves convincing results and, together with acidic corrosion inhibitors from corresponding sources, completely renewable systems for corrosion inhibition are provided.

BACKGROUND OF THE INVENTION

The use and manufacture of metalworking fluids (MWFs) represent a majorcost-factor in the industrial processing of metals. MWFs play asignificant role in processes like drilling, forming, grinding, orcutting of metals. Besides their lubricating capabilities, in theseapplications MWFs are used as coolants. This is achieved by conductingand dissipating accruing heat and reducing friction between a work pieceand the tool and, therefore, influence the heat generation. MWFs are notonly used for iron and steel but also for processing light metals, suchas aluminum, magnesium, and their alloys, and are crucial in order toavoid thermal damage of the material of the work piece and to reducewear on the tool.

MWFs can be divided in three groups: Oil-based, water-based, andemulsions. Water-based formulations may be composed in form of fairlycomplicated solutions containing up to 300 different additives. Oneimportant group of such additives are acidic corrosion inhibitorsincluding but not limited to sulfonates, organic boron compounds, fattyacids, carboxylates, and phosphonates. For their use in neutral oralkaline media they are usually composed of an acid and a base toneutralize the acid. Typically, amino alcohols are used for such aneutralization. Besides 2-Amino-2-methylpropanol (AMP) and1-Aminopropan-2-ol (MIPA). Triethanolamine (TEA) is one of thepredominantly used bases for this purpose. However, despite itscomprehensive usage TEA has some major downsides. In the past it hasbeen linked to allergic contact dermatitis, seawater ecotoxicity, andcarcinogenic activity. Additionally, it is listed on the “EU ControlList of Dual-Use Items”, which entails further regulation and higherexpenses.

Thus, there is a general need for the replacement of commonly used aminoalcohols like TEA with more environmentally friendly and less toxicbases for the neutralization of acidic corrosion inhibitors to be usedas components of MWFs. While some attempts to utilize simple amino acidsas “green” acidic corrosion inhibitors may have been made, salts ofamino acids as neutralizing agents for acidic corrosion inhibitors havenot been used so far. Proteinogenic as well as other amino acidsare—unlike TEA and other commercially used organic amines and aminoalcohols—nontoxic and non-hazardous. Additionally, they are easilyavailable from renewable sources.

DETAILED DESCRIPTION OF THE INVENTION

With the present invention it has been found that the salts of baseswith readily available amino acids (including proteinogenic aminocarboxylic acids, such as methionine, cysteine, glutamic acid, arginine,and glycine, analogues and derivatives thereof, such as aminocaproicacid, as well as amines with other acid functionalities like a sulfonicacid group, a phosphonic acid group, or a phosphoric acid group, such astaurine) and dipeptides (such as N-(L-α-aspartyl)-L-phenylalanine or its1-methyl ester), which are nontoxic, nonvolatile, and available in bigtonnages from bio-renewables, can be used as highly efficient greenneutralizing agents for corrosion inhibition in alkaline media—replacing(fully, wherein the amino acid represents the only neutralizingcomponent, or in part) environmentally problematic TEA and similarorganic amines that are each, for several reasons, environmentallyproblematic.

With the present invention it was surprisingly found that the use ofamino acids (such as methionine, cysteine, glycine, glutamic acid,serine, arginine, histidine, alanine, lysine, aminocaproic acid, ortaurine; each used in their deprotonated form—e.g. in form of solutionsof their sodium, potassium, or lithium salts)—wherein the amino acid ismonomeric or in form of a dipeptide or wherein the amino acid is aderivative of a monomeric amino acid or of a dipeptide—in theneutralization of acidic corrosion inhibitors leads to very effectiveanticorrosive systems that are, in view the aforementioned problems ofcommonly used bases, highly advantageous. In comparison to mixtures ofthe same acidic corrosion inhibitors with the standard neutralizingagent triethanolamine (TEA), the amino acids of the present inventionprovide at least similar—in several cases increased—efficacies. Theamino acids can be formulated with a broad range of acidic corrosioninhibitors including aliphatic, cycloaliphatic, and aromatic carboxylicacids, sulfonates, phosphonic and phosphoric acids, boric acid, tall oilderived acids, and others. Typical examples of acidic corrosioninhibitors to be used with the present invention are azelaic, sebacic,undecanoic, and dodecanoic acid, triazintriyltriiminotrihexanoic acid(TC®) arylsulfonamido carboxylic acid (ASCplus®), 11-phosphonoundecanoicacid (PUDA), phosphonobutanetricarboxylic acid (PBTC),amino-tris(methylenephosphonic acid)-N-oxide (ATMP-N-Oxide), orhexamethylenediaminetetra (methylenephosphonic acid) (HDTMP) but alsophosphonic acid derivatives of terpenes and fatty acids, such asphosphonic acids derived from geraniol, citronellol, or pinene.

Preferably the amino acid comprises a primary amino-function. Thepreferred amino acids of the present invention are proteinogenic aminoacids, β-alanine, γ-aminobutyric acid, aminocaproic acid and taurine;most preferred are glycine, alanine, lysine, methionine, taurine, andaminocaproic acid (ACA).

In view of the aforementioned advantages of amino acids, which arenon-toxic and available from renewable sources, and in view of theirsurprising efficacy in mixtures with acidic corrosion inhibitors, thepresent invention provides highly advantageous anticorrosive additivesfor MWFs that can be used in the industrial processing of numerousmetals. It was well noted that neutralized amino acids and acidiccorrosion inhibitors can easily be formulated into stable solutions inwater, which can be stored at room temperature for months. In addition,it is remarkable that highly concentrated compositions can beformulated, such as, for example, 98 g of TC®, 80 g of amino caproicacid, and 24.4 g of NaOH in 100 ml of water or 84.6 g of TC®, 39.1 g ofglycine, and 22.1 g of NaOH in 100 ml of water

Comparisons of the commonly used neutralizing agent, TEA, with theneutralizing agents in accordance of the present invention, amino acids,which are used in deprotonated form (in form of their salts or inequimolar mixtures with, e.g., NaOH), showed that the latter may furtherincrease the efficacy of the employed acidic corrosion inhibitors. Asknown from the art, simple inorganic bases, such as NaOH, that are usedto neutralize acidic corrosion inhibitors do not result in efficientanti-corrosion systems, and while we do not want to be bound to thistheory, it is expected that the amino acids interfere with the metalsurfaces on their own and, thus, stabilize the layer formed by theprimary acidic corrosion inhibitor. In particular, glycine, taurine, andACA lead to highly efficient anticorrosive systems. While the aminoacids are already effective at low concentrations (such as one molarequivalent, relative to the amount of the acidic corrosion inhibitor andthe number of acid functions in the acidic corrosion inhibitor) andwhile the amino acids may be used also in higher concentrations, thepreferred molar ratios of the amino acid (with one equivalent of base,such as NaOH or KOH) and the anticorrosive acid is 1/1-3/1 per acidgroup of the anticorrosive acid; preferably, the ratio is 1.5/1 per acidgroup of the anticorrosive acid. E.g., in the case of TC containingthree carboxylic acids, the preferred ratio is 4.5 equivalents of theamino acid and NaOH—thus, the absolute molar ratio of the components(amino acid/Na0H/TC) is 4.5/4.5/1.

As demonstrated in the Chip-Filter-Test, the amino acids of the presentinvention provide highly effective anti-corrosion systems for steel. Inaddition, in Leaching-Tests with Co, Cu, and Ni it was demonstrated thatthe amino acids in accordance with the present invention prevent or, atleast, limit the undesired leaching of alloy elements and that they canbe used successfully when handling, for example, Co-, Cu-, and/orNi-containing steel-alloys; in particular, ACA shows impressive resultsoutperforming even the current gold-standard for low Co-leaching,applications, MIPA. Therefore, the amino acid-additives of the presentinvention allow to reduce the undesired leaching of certain elements(such as Co, Cu, and Ni) from an alloy, which is regularly observed whena corresponding workpiece is processed in the presence of a commonmetalworking fluid that comprises, e.g., TEA as basifying component.

Corrosion inhibition is not only important when handling iron or steel,it is also important when processing light metal alloys of, e.g.,aluminum or magnesium. While commonly used carboxylic acid additiveslike TC are not capable of preventing corrosion or staining on lightmetal alloys, mixtures of octylphosphonic acid (OPA) and TEA providesome corrosion inhibition on aluminum. In the corresponding tests (witha copper containing hardened Al-alloy used in aerospace applications, AL5083, an aluminum-alloy mostly used for welding and marine applications,AL 2024, and a standard wrought magnesium-alloy containing 3% aluminumand 1% zinc used for example in automotive industry, MG AZ31), it wassurprisingly found that the amino acids of the present invention (suchas glycine or ACA, each with an equimolar amount of NaOH) can act asfull and extremely valuable substitutions of the environmentallyproblematic TEA. In case of AL 5083 and MG AZ31, the amino acids of thepresent invention showed some effect even with TC and the mixtures ofthe present invention outperformed those of the prior art. Additionally,in view of the results achieved with aluminum and magnesium, it canplausibly be expected that the amino acids of the present invention canalso be used to neutralize acidic corrosion inhibitors, when formulatingefficacious anticorrosive systems for the processing of other metals andalloys, such as titanium, beryllium, and zirconium.

With the present invention it has been demonstrated that in combinationswith acidic corrosion inhibitors, amino acids, which are highlybeneficial due to their general biocompatibility, cost effectiveness,and nontoxicity, allow to replace harmful and problematic TEA—inapplications on steel, aluminum, and magnesium.

The amino acids of the present invention are compatible with a range ofacidic corrosion inhibitors, such as phosphonic and/or carboxylic acidscommercially used for steel and aluminum. These new systems achievecorrosion scores as good as or even better than the corresponding TEAbased additives of the prior art. Additionally, with the presentinvention anticorrosive systems entirely based on renewable resourcesbecome available—e.g., by using the amino acids of the present inventionas neutralizing agents for natural product-derived acidic corrosioninhibitors, such as geranylphosphonic acid, pinene derived phosphonicacid (PDPA), or 11-phosphonoundecanoic acid (PUDA). These fullyrenewable mixtures are highly efficient anticorrosives and can beutilized in the processing of steel, aluminum, and even magnesium. Thecorrosion inhibitors of the present invention fit the ongoing need ofgreen chemistry to develop technologies and materials that areintrinsically nontoxic to living organisms and the environment and thatminimize harmful waste.

EXPERIMENTAL SECTION

Syntheses

11-Phosphonoundecanoic acid (PUDA) may be synthesized from undecylenicacid via palladium-catalyzed hydrophosphorylation with H₃PO₂.Geranylphosphonic acid can be synthesized via palladium catalyzeddehydrative allylic substitution with H₃PO₂ and geraniol and subsequentoxidation with iodine and DMSO. PDPA may be obtained via radicaladdition of ammonium hypophosphite with triethylborane to β-pinene andsubsequent oxidation with iodine and DMSO (scheme 1).

Chip-Filter-Test

The evaluation of the rust preventive properties of water-misciblecoolants was conducted with 2 mL 3 w.-% solutions (calculated to theacid) and 2.0 g sieved grey cast 25 chips for 2 h according to DIN51360-02-A.

TABLE 1 Results of the Chip-Filter-Test of TC (3 w.-%) neutralized withTEA or amino acids of the present invention (in form of their sodiumsalt). Entry Base Eq. pH Corrosion Score 1 TEA 3.6 7.4 0 2 NaOH 3.6 10.34 3 KOH 3.6 9.1 4 4 Taurine 3.6 8.5 1 5 Glutamic acid* 3.6 9 1 6Arginine 3.6 9.5 0 7 Histidine 3.6 7.5 1 8 ACA 3.6 11 0 9 TEA 4.5 7.8 010 Methionine 4.5 8.9 0-1 11 Glycine 4.5 9.7 1 12 Taurine 4.5 8.65 0-113 Glutamic acid* 4.5 9.1 1 14 ACA 4.5 10.6 0 15 Glycine 5.0 9.62 0*Used as disodium salt.

TABLE 2 Conditions and corrosion scores of Chip-Filter-Tests of variousacidic corrosion inhibitors (acid, each in a concentration of 3 w.-%) incombination with TEA or an amino acid for neutralization. CorrosionScores/Bases Entry Acid Eq. of TEA Glycine* Taurine* ACA* 1 TC 4.5 0 0 00 2 ASC PLUS 2.5 0 3-4 2-3 0 3 Sebacic 3 0 0 1 0 acid 4 Undecanedioic 30 0 0 0 acid 5 Dodecanedioic 3 0 0 0 0 acid 6 PUDA 3.6 0 0 0 0 7 PUDA4.5 0 0 0 0 8 Geranyl- 3 0 0 0 0 phosphonic acid 9 ATMP- 5 0 0-1 0-1 0N-Oxide *Used as Sodium salt. ** 4.5 eq of amino acid and 4 eq of NaOH.***after 16 h at 60° C.

Staining Assay. The evaluation of corrosion inhibition for light metalalloys was conducted with 2.5 mL of an aqueous solution containing up to4 weight % of the acidic corrosion inhibitor and 1*3 cm plates ofaluminum or magnesium alloys for 24 h at 40° C. The plates were placedin a small glass vessel with half of their surface being covered by thetest medium. Subsequently, the staining was rated on a scale from 0 (NoStaining) to 4 (strong staining). The optical assessment of staining wasperformed in accordance to Watanabe et al. (S. Watanabe. J. Oleo Sci.2008, 57, 1-10).

TABLE 3 Conditions and scoring of TC, PUDA, PDPA and OPA in combinationwith TEA, Glycine and ACA on Al 2024. Performed in hardwater (10° dH) at40° C. for 24 h. Staining Scores/ Aluminum 2024 Concentration of AcidEntry Acid Eq. of Base Base 2% 3% 4% 1 TC 4.5 TEA 3 3 2 2 PUDA 4.5 TEA0-1 0 0 3 PUDA 4.5 Gly + NaOH 1 0 0 4 PUDA 4.5 ACA + NaOH 1 0 0 5 PDPA3.0 TEA 0 0 6 PDPA 3.0 Gly + NaOH 0 0 7 PDPA 3.0 ACA + NaOH 0 0 8 OPA3.0 TEA 0 0 0 9 OPA 3.0 Gly + NaOH 0 0 0

TABLE 4 Conditions and scoring of TC, PUDA, PDPA and OPA in combinationwith TEA, Glycine and ACA on Al 5083. Performed in hardwater (10° dH) at40° C. for 24 h. Staining Scores/ Aluminum 5083 Concentration of AcidEntry Acid Eq. of Base Base 2% 3% 4% 1 TC 4.5 TEA 3 3 3 2 TC 4.5 ACA +NaOH 2 2 2 3 PUDA 4.5 TEA 3 0 0 4 PUDA 4.5 Gly + NaOH 2 1 0 5 PUDA 4.5ACA + NaOH 0 0 0 6 PDPA 3.0 TEA 0 0 7 PDPA 3.0 Gly + NaOH 0 0 8 PDPA 3.0ACA + NaOH 0 0 9 OPA 3.0 TEA 0 0 0 10 OPA 3.0 Gly + NaOH 0 0 0

TABLE 5 Conditions and scoring of TC, PUDA, PDPA and OPA in combinationwith TEA, Glycine and ACA on Mg AZ31. Performed in hardwater (10° dH) at40° C. for 24 h. Staining Scores/ Magnesium AZ31 Concentration of AcidEntry Acid Eq. of Base Base 2% 3% 4% 1 TC 4.5 TEA 4 4 4 2 TC 4.5 Gly +NaOH 3 3 3 3 TC 4.5 ACA + NaOH 3 3 3 4 PUDA 4.5 TEA 4 3 3 5 PUDA 4.5Gly + NaOH 4 4 2 6 PUDA 4.5 ACA + NaOH 4 1 0 7 PDPA 3.0 ACA + NaOH 3 1 8OPA 3.0 TEA 2 2 2 9 OPA 3.0 Gly + NaOH 4 2 2 10 OPA 3.0 ACA + NaOH 3 3 2

Leaching

Another parameter to consider for the industrial usage of metal workingfluid is their leaching characteristics for important alloy metals suchas Co, Ni or Cu. AMP and MIPA are marketed as low leaching additives.Several amino acids were examined. For realistic results, the solutionwas pH-adjusted to match the pH value of the mixture in theChip-Filter-Assay.

225 mg Co, Ni or Cu-Powder (<150 μm, 99.9% trace metal basis) wassuspended in 15 ml of a 1 w.-% aqueous solution of the amine (in case ofthe amino acids and taurine, the pH value was adjusted with sodiumhydroxide) and heated under reflux for 24 hours. The suspension wascooled to room temperature, filtered and the metal-concentration (eitherCo, Ni or Cu) of the filtrated was measured directly via F-AAS.

TABLE 6 Detected Co, Ni and Cu-concentrations and pH values for eachtested amine. The concentration was measured from the filtrate via F-AASConcentration Co ∅ Concentration Cu ∅ Concentration Ni ∅ Amine F-AAS(mg/L) F-AAS (mg/L) F-AAS (mg/L) — 0.67 ≤0.10 ≤0.01 TEA 34.3 2.80 1.44AMP 7.03 42.9 0.33 MIPA 0.82 26.0 1.26 ACA* ≤0.20 0.81 ≤0.01 *Used assodium salt.

1. Method of of neutralizing an acidic corrosion inhibitor in theformulation of a metalworking fluid, which comprises incorporating amamino acid as a neutralizing component in said metalworking fluidwherein the amino acid is used in deprotonated form as its salt with abase; preferably in form of its alkali metal- or earth alkali metalsalts, which may be formed in situ in an equimolar mixture with NaOH,KGH, or LiOH, and wherein the amino acid is monomeric, in form of adipeptide or wherein the amino acid is a derivative of a monomeric aminoacid or of a dipeptide.
 1. The method of claim 1, wherein the amino acidrepresents the only neutralizing component and/or wherein the amino acidcarries a primary amino-function.
 3. The method of claim 1, wherein themonomeric amino acid or both components of the dipeptide areproteinogenic amino carboxylic acids, analogues or derivatives thereof,or amines with another acid functionality.
 4. The method of claim 1,wherein a. the monomeric amino acid is glycine, methionine, cysteine,amino caproic acid (ACA), glutamic acid, or taurine, or the dipeptide isN-(L-α-aspartyl)-L-phenylalanine or its 1-methyl ester, b. the acidiccorrosion inhibitor is triazintriyltriiminotrihexanoic, arylsulfonamidocarboxylic acid, sebacic acid, undecanoic acid, dodecanoic acid, azelaicacid, tall oil derived acids, sulfonates and/or phosphonic—andphosphoric acids, and for c. the metal is iron, aluminum, magnesium,titanium, beryllium, zirconium or a corresponding alloy (such as Al5083, Al 2074, or Mg AZ31).
 5. The method of claim 1, wherein the aminoacid is present in an amount of one molar equivalent or more (relativeto the number of acid functions of the acidic corrosion inhibitor),preferably in an amount of 1-3 molar equivalents, most preferably in anamount of 1.5 molar equivalents.
 6. The method of claim 1 when used forlimiting the leaching of alloy elements (such as Co, Cu, and Ni) from acorresponding alloy.
 7. Metalworking fluid comprising an acidiccorrosion inhibitor and an amino acid, wherein the amino acid ismonomeric, in form of a dipeptide or wherein the amino acid is aderivative of a monomeric amino acid or of a dipeptide, and wherein theamino acid is used in deprotonated form as its salt with a base.
 8. Themetalworking fluid of claim 7, wherein the amino acid is glycine,methionine, cysteine, aminocaproic acid (ACA), glutamic acid, ortaurine, or a salt thereof, preferably the sodium, potassium, or lithiumsalt.
 9. The metalworking fluid of claim 7, wherein the amino acid ispresent in an amount of one molar equivalent or more (relative to thenumber of acid functions of the acidic corrosion inhibitor.
 10. Themetalworking fluid of claim 7, wherein the acidic corrosion inhibitor isa phosphonic acid derivative of a terpene or a fatty acid.
 11. Themetalworking fluid of 7 which is water-based, oil-based, or in form ofan emulsion.
 12. The metalworking fluid of claim 7 further comprising asynthetic amine, such as 2-amino-2-methy Ipropanol 1 (AMP),1-aminopropan-2-ol (MIPA) or triethanolamine (TEA).
 13. Method ofprocessing of iron, aluminum, magnesium, titanium, beryllium, zirconiumor a corresponding alloy (such as Al 5083, Al2024, or Mg, AZ31) whichcomprises using a metalworking fluid as claimed in claim
 7. 14. Themethod of claim 3, wherein said amine with another acid functionalitycontains a sulfonic acid group, a phosphonic acid group, or a phosphoricacid group.
 15. Metalworking fluid of claim 7, wherein said amino acidis used in form of its alkali metal- or earth alkali metal salts or inform of an equimolar mixture with NaOH, KOH, or LiOH.
 16. Metalworkingfluid of claim 15, wherein said amino acid is used in the form of itssodium, potassium, or lithium salt.
 17. Metalworking fluid of claim 9,wherein the amino acid is present in an amount of of 1-3 molarequivalents, most preferably in an amount of 1.5 molar equivalentsrelative to the number of acid functions of the acidic corrosioninhibitor.
 18. The metalworking fluid of claim 8, wherein the amino acidis present in an amount of one molar equivalent or more (relative to thenumber of acid functions of the acidic corrosion inhibitor. 19.Metalworking fluid of claim 18, wherein the amino acid is present in anamount of of 1-3 molar equivalents, most preferably in an amount of 1.5molar equivalents relative to the number of acid functions of the acidiccorrosion inhibitor.
 20. Method of_processing of iron, aluminum,magnesium, titanium, beryllium, zirconium or a corresponding alloy (suchas Al 5083, Al2024, or Mg AZ31)_which comprises using a metalworkingfluid as claimed in claim 18.